I'm currently doing a small fitness regimen that includes ten exercises aimed at strengthening the core of the body.
The core exercises are as follows: basic crunch, crunch on a stability ball, reverse crunch, forearm plank, forearm side plank, Russian twist, back extension, Superman, bicycle crunch, and jack knife.
Unfortunately an old shoulder/back injury makes the jack knife too painful for me to do. I've read on the forums that a plank can be an acceptable substitute for the jack knife. Should I do more of those each workout, or are there other substitutes that are not already incorporated into my core workout?
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